1. Field of the Invention
The present invention relates to recording devices, such as magnetic disk devices, and, more particularly, to a head positioning device.
2. Description of the Prior Art
A magnetic disk device will be taken as an example of the recording devices, and the positioning of the magnetic head will be described below.
There are two modes of positioning the magnetic head. One mode is to move the magnetic head over a number of tracks. It is desired that the magnetic head reaches the target track as fast as possible and that when it reaches the target track, the moving speed is almost zero. The other mode is to make the magnetic head follow a specific track. The tracking error of the magnetic head should be small.
A conventional head positioning device is shown in FIG. 10. It includes magnetic disks 10 as a recording medium; a servo surface 1 on which positioning information is recorded; a servo head 2 for reading the positioning information from the servo surface 1; data surfaces 5; data heads 6; a position error detecting circuit 3 for processing the positioning information from the servo surface 1 to generate position signals 45 and 46; a position controlling circuit 4 in response to the position signals 45 and 46 to generate a linear signal 41; a position correcting circuit 8 in response to the linear signal 41 to generate a feedback signal 12 for controlling the position and speed of the magnetic head; a power amplifier 9 for amplifying the feedback signal 12; an actuator 11 for moving the magnetic head; and a motor 13 for moving the actuator.
The manner in which the position error detecting circuit 3 responds to the positioning information read from the servo surface 1 to generate position signals 45 and 46 will be described with reference to FIG. 11. Characters N and S represent north and south magnetic poles recorded on the servo surface 1, respectively. A servo pattern 61 consisting of these Ns and Ss forms positioning information. Data tracks 83a, 83b, . . . , 83e, which do not actually exist on the servo surface, are shown by broken lines in order to indicate the positional relationship to the servo tracks 63a, 63b, . . . , 63e which are offset a half track from the data track.
Waveforms 62a and 62b are read out by scanning the servo pattern 61 in the direction of an arrow with the servo head 2 at positions 2a and 2b, respectively. The signal 45 is a position signal A which is a difference between the E position signal and the F position signal of the readout waveforms 62a and 62b with the servo head at respective positions. The signal 46 is a position signal B which is a difference between the G position signal and the H position signal of the readout waveforms 62a and 62b, with the servo head at respective positions.
In this way, the servo head 2 generates the waveform 62 based on the N-S pattern of a servo track 63. When the signal differences (E-F) and (G-H) are computed based on respective head positions, the resulting positioning signals A and B are offset by a quarter period with respect to each other. These signals are inputted to the position control circuit 4.
The operation of the two-phase servo system, in which two positioning signals are offset by a quarter period, will be described with reference to FIGS. 12 and 13.
FIG. 12 shows in block form a conventional position control circuit. As has been described above, the positioning information or servo pattern 61 written on the servo surface 1 is read by the servo head 2 and converted by the position error detecting circuit 3 into continuous and periodic positioning signals A and B which are offset by a quarter period from each other. The positioning signals 45 and 46 are processed in the comparative operation circuit 15 to generate a window signal 42. With this window signal 42, one of the positioning signals 45 and 46 or their inverted signals 55 and 56 is outputted as a linear signal 41.
The operation will be described with reference to FIG. 13. All the areas on tracks 83a, . . . , 83f are divided into two sections by the fact that the position signal A is greater than the position signal B as shown by a logic signal X. Similarly, when they are divided by the fact that the position signal A is greater than the inverted position signal B, a logic signal Y offset by a quarter period from the logic signal X is obtained. Four window signals 42a-42d are generated by combinations of these logic signals, positive and negative.
These processes are carried out by the comparative operation circuit 15 of FIG. 12 An analog switch 16 outputs as a linear signal 41 only a signal selected by the window signal 42 from four signals; namely, the position signal 45, its inverted signal 55 from the inverter circuit 51, the position signal 46, and its inverted signal 56 from the inverter circuit 52. The position correcting circuit 8 generates a feedback signal 12 so that the linear signal 41 becomes zero. In this way, wherever the magnetic head is, the linear signal indicative of a positional error from the closest track center is used for a closed loop control.
That is, the linear signal 41 is converted into the feedback signal 12 by the position correcting circuit 8 and applied to the power amplifier 9, wherein the positional error is converted into an electric current, which is applied to the head drive motor 13. The head drive motor 13 generates a force corresponding to the intensity of the electric current to drive the actuator 11 to which the magnetic head is attached. With such a closed loop control, the magnetic head is positioned at the center of a target track.
In order to make the magnetic head follow a track, a sector servo system is used in the magnetic disk device. In the sector servo system, the data tracks of a data surface are divided into a number of sectors, and position correcting information is recorded in each sector. The position correcting information is read out to correct the position of a magnetic head.
In general, one of the most difficult problems with recording and reproducing in a fixed or floppy magnetic disk device is to position the magnetic head accurately. That is, when data is reproduced with a magnetic head which is out of track caused by an external cause, the signal-to-noise ratio of an analog signal becomes low under the influence of a crosstalk with adjacent tracks or unerased residue data, thereby increasing the probability of a data error. Also, when data is recorded with the out-of-track head, a similar data error can take place. Most of the out-of-track errors are caused by mechanical or thermal errors. The representative errors include errors made in installation of the magnetic recording disks, the magnetic heads, and the magnetic head positioning mechanism, differences in thermal coefficients thereof, and a fall of the motor shaft for rotating the magnetic recording disks. Under these mechanical and thermal influences, the magnetic head moves out of track. This is generally called "off track."
In order to increase the magnetic recording density, it is necessary to increase either the bit density in the circumferential direction or the track density in the radial direction. In order to increase the track density, it is essential to reduce the off track. The degrees of the mechanical and thermal influences increase with the track density. For this reason, recent models employ a sector servo system to reduce these influences. In the sector servo system, position correcting information for correcting the off track is recorded at either the leading or trailing portion of each sector so that the data head reproduces the position correcting information to detect the amount of off track and moves the data head to the track center, thus providing the correct positioning.
FIG. 14 is helpful for explaining the sector servo system. The data surface 5 of a magnetic disk 10 is divided into n sectors S.sub.0, S.sub.1, . . . , S.sub.n-1. Each sector S.sub.i consists of a data area 81 in which the data is stored and a position correction information area 80 in which information for correcting the head position is recorded. The magnetic disk rotates in the direction of an arrow R so that each sector S.sub.i passes below the data head 6, which is movable in the radial direction of the magnetic disk, so that each piece of information is written or read. The data tracks 83 are formed concentrically and have an equal width. In order to write or read data, it is desired that the center of the magnetic head follows the center line of a data track 83 with few errors. The position correction information area 80 consists of an erasing portion 84, a synchronizing portion 85, and a position information portion 86. The position information portion 86 is used to make the data head 6 follow the track 83.
Position correcting information 32 and 33 is written in the position information portion 86 as shown in FIG. 15 to detect position errors (hereinafter "stack errors") of the servo head 2 and the data head 6 as shown in FIG. 16. For the data head 6a, a difference between the signal readout waveform 34a of the position correcting information 32 and the signal readout waveform 35a of the position correcting information 33 is converted into the amount of off track by the off-track detecting circuit 7 and applied to the position control circuit 4 and the position correction circuit 8. The output of the power amplifier 9 is supplied to the motor 13 to drive the actuator 11 so that the data head 6a is positioned at the center of a data track. When the data head 6c is selected, a difference between the signal readout waveforms 34b and 35b is taken to correct the head position in the same way as for the data head 6a.
As shown in FIG. 17, the servo pattern 61 and the position correction information 32 and 33 as described above are written by a dedicated writing device or servo writer 91 with the dust cover 93 removed. As shown in FIG. 18, then the dust cover 93 is attached to be ready for use. However, the mechanical distortion caused by the installation brings about a stack error and other causes for reducing the positioning precision.
In the conventional magnetic disk device, the head is positioned at the track center so that either of the positioning signals 45 and 46 becomes zero. Consequently, in order to increase the track density (or reduce the track width), it is necessary to reduce the width of positioning information written on the servo surface and the width of the servo head to read the information. For this reason, the influence of a medium defect of the magnetic disk becomes so large that it reduces the precision of head positioning.
In addition, the magnetic disk devices using the conventional sector servo system require a special instrument to write the position correction information. Also, the positional relationship between the servo head and the data head is changed by the mechanical distortion caused by the assembly of the dust cover after the position correction information is written by the installation of the device on the system housing, thereby causing a stack error. Another stack is caused by the atmospheric changes such as temperature changes, thus increasing the amount of position correction of the data head after the positioning of the servo head. For these reasons, the positioning precision becomes very low or it takes a long time to position the magnetic head accurately.